This invention relates generally to flat panel display devices and particularly to an electron beam injection structure for such devices.
Prior art flat panel display devices, such as those described in U.S. Pat. Nos. 4,234,815 and 4,263,529 include a baseplate and a faceplate which are held in a spaced parallel relationship by a plurality of external side walls. The envelope is divided into a plurality of channels by internal walls which also provide support against collapse due to atmospheric pressure. Each of the channels includes a guide mesh structure, such as that described in U.S. Pat. No. 4,330,735, to propagate electron beams the lengths of the channels until one line of the visual display is to be produced.
The beam guide assemblies include beam guide meshes, a focus mesh and an acceleration mesh all of which are arranged in a spaced parallel relationship. The electrons are emitted from a cathode which is arranged at the entrance to each guide mesh structure. The guide mesh structures are biased positive with respect to the cathode and thus control the injection of electrons to the guide mesh structure from the cathode. A plurality of extraction electrodes is arranged on the baseplate and a phosphor screen, which luminesces when struck by electrons, is arranged on the faceplate. The meshes of the beam guide assembly include apertures arranged in columns longitudinally along the meshes and when a three-beam system is used are arranged in rows transversely of the meshes. The guide mesh assemblies are disposed with the apertures in the guide meshes in alignment with the extraction electrodes. Electrons are propagated in the space between the guide meshes by biasing the focusing mesh and the extraction electrodes to focus the electrons into beams between the two guide meshes. When a line is to be produced on the screen, an appropriate extraction electrode is biased with a negative voltage to repel the electrons through the apertures to propagate toward the screen. The acceleration mesh is positively biased to attract the electrons toward the screen. Flat panel display devices built in accordance with the prior art operate satisfactorily for the purposes intended. In the prior art devices the beam guide meshes between which the electron beams propagate are biased positive with respect to the cathodes. This positive voltage on the beam guide meshes causes the electrons emanating from the cathodes to be injected into the space between the guide meshes. The efficiency; that is, maximum electron beam current and minimum electron leakage, of electron injection into the beam guide meshes can be enhanced by changing the positive voltage on the guide meshes. Electron leakage results in stray electrons randomly reaching the screen and lowering the contrast of the visual display. Enhancement of the efficiency of electron injection into the beam guides is desirable because the lower electron leakage improves the contrast of the visual display. However, changing the guide mesh voltage results in the requirement that other operating parameters, such as the focus voltage and the anode voltage, also are changed. Typically the other operating parameters are selected to optimize other characteristics, such as electron spot size, of the display device. For this reason, it is highly undesirable to change these parameters and, therefore, the efficiency of electron injection to the guide meshes cannot be optimized. It, therefore, would be advantageous to efficiently inject electrons from the cathode into the space between the guide mesh pairs independently of the other operating parameters of the display device.
The instant invention provides this advantage by the provision of an electron beam injection structure which maximizes the injection of electrons into the beam guide structure independently of the voltage on the beam guide meshes and of the other operating parameters of the display device.